The present invention relates to a method and an apparatus for detecting a focus condition of an objective lens for use in a still camera, a cinecamera, an optical apparatus for taking microphotographs, etc.
Until now various types of the focus detection methods and apparatuses have been proposed. For example, in a Japanese Patent Laid-open Publication No. 60,645/73, the apparatus has been proposed wherein light fluxes transmitted through symmetric portions of an objective lens with respect to its optical axis are received by light receiving elements respectively through a vibrating lattice and a focus condition of the lens is detected on the basis of a difference between photoelectrically converted outputs derived from these light receiving elements. However, in such focus detection apparatus, the lattice must be supported movably and must be vibrated by a driving system, a whole apparatus becomes large in size and complex in construction.
In order to eliminate such drawbacks, another known detection apparatus has been further proposed in a Japanese Paten Laid-open Publication No. 142,512/77, wherein an image to be inspected is divided into two images by means of two wedge-shaped prisms arranged symmetrically and the two images are projected onto two series of photo receiving elements through a relay lens and a focusing condition is detected on the basis of a lateral shift of the divided images by suitably processing output difference between these series of light receiving elements. In the focus detection apparatus mentioned above, the system construction can be made relatively simple because it is not necessary to provide the lattice vibrating system and the movably supporting mechanism. However, since the images divided by the wedge-shaped prisms are projected onto the two series of light receiving elements by means of the relay lens, it is not possible to satisfy sufficiently a requirement for a compact system and is extremely difficult to adjust an optical positional relation between the relay lens, the series of the light receiving elements, etc. Moreover, since use is made of the two series of light receiving elements which are arranged side by side, it is not possible to perform the focus detection in a highly accurate manner due to differences of their characteristics.